Solder stencils are generally utilized in the manufacture of contacts on printed circuit boards to define the shape of a solder paste on the surface to be soldered. Solder paste is a suspension of flux and small spheres of solder particles. When heated, the paste forms a solder which secures two metal pieces together. Solder paste is screen printed onto a circuit board using a stencil. The stencil is a thin sheet of metal that has a desired hole pattern formed in the stencil.
In a reflowing process, the solder melts, forming a mechanical and electrical connection between printed circuit board land pads and an electrical component. Flux encourages the formation of this connection. Flux may be removed through a cleaning process or left on the board with the so-called no clean process.
One problem which may occur with solder paste is the formation of solder balls. Solder balls are small spherical solder particles which are formed away from the main solder pool during reflow. If these balls become loose at any time after the product is manufactured, they can roll to a position where they can short an electrical component. Solder balling may occur around components and over the board. Large solder balls may be associated with small and low clearance passive components such as 0805 and 1206 resistor and capacitor connection sites. The large solder balls may not be removed in the cleaning process and will persist with the no-clean process.
In addition, excessive solder balling may prevent the solder from making a good solder joint fillet. Thus, the formation of solder balls may create reliability issues. There has been a continuous effort to minimize the occurrence of solder balling. See C.A. Harper, Electronic Packaging & Interconnection Handbook, 2d Edition, McGraw Hill (1997), at p. 5.54.
It would be highly desirable to have a technique for forming soldered connections using solder paste which reduces the occurrence of solder balling.